Function oniro_seed 

pub const fn oniro_seed(replicate_seed: u64) -> i64

Reduces a u64 replicate seed to a non-negative i64, so it fits oniro’s TOML integer run.seed field.

The reduction masks off the sign bit (seed & i64::MAX), which is deterministic: equal input seeds map to equal run.seed values, so paired (common-random-numbers) seeds stay common across trials. Two distinct u64 seeds could in principle collide after masking, but only across the 63-bit boundary and with probability 2^-63 — irrelevant to the pairing property.